The present invention relates to measurement of stress in rock, and more particularly to a jack fracturing method and apparatus for measuring the natural state of stress of rock, as well as the value of Young's Modulus for an assumed value of Poisson's Ratio of the rock.
In structural engineering, it is imperative that the natural state of stress in rock masses at a preselected depth be known for rational design of earth supported structures. Knowledge of the in situ earth stresses is also of importance in the design of tunnels, mines, dams and other subterranean structures.
The present techniques for measuring the state of stress of rock in situ include: (a) the rock property method, (b) the rock strain method and (c) the rock fracture method.
In the rock property method, a property of the rock material, such as sound propagation velocity, is measured, and the property is correlated to rock stress. However, in practice, only two normal propagation velocities are detectable at a preselected region in the rock, and the data derived therefrom are inadequate to determine the six independent components of stress that exist in the general case.
In the rock strain method, a rock segment is stress relieved in situ by cutting it free from its surroundings. This is typically done by drilling a strain relief borehole adjacent a primary borehole, and the change of strain of the rock segment between the boreholes is measured. The change of strain is related to the initial state of stress of the rock and can be determined using tensor analysis. The rock strain method is widely used because it provides the magnitudes as well as directions of the principle stresses of the rock. However, the rock strain method is impracticable for deep subterranean regions stress determination because the strain-relieving borehole must have a precise orientation with respect to the primary borehole. A technique known as overcoring is commonly used whereby a tubular strain-relieving borehole is cut coaxially to the primary borehole to relieve the stress on the primary borehole wall. Obviously, alignment of the two boreholes is critical and the rock strain method is practical only in relatively shallow regions.
On the other hand, the rock fracture method is somewhat practical for deep borehole strain determination. There, a non-compressible fluid is sealed into an enclosed portion of the borehole. The hydraulic pressure in the sealed portion is increased using a hydraulic pump until the wall of the enclosed portion of the borehole is caused to fracture. The magnitude of the hydraulic pressure required to fracture the borehole wall is determinative of two components of stress. But since nine stress components exist in the general case, the borehole must be oriented in a principle stress direction in order to eliminate the shear stress components. Precise orientation of the borehole is somewhat difficult to achieve in deep borehole regions, and the measurements are very sensitive to the porosity of the rock. Variations in rock porosity and misorientation of the borehole with respect to a principle stress direction diminish the accuracy of the stress measurements.
It is well known that stress and strain are related to each other in a material as a function of Young's Modulus, (modulus of elasticity), E, for a given value of Poisson's ratio .mu.. Typically, E is measured in rock by extracting a sample of the rock from the region of interest and then studying the stress-strain behavior of the sample. Thus, determination of the natural state of stress of the rock requires two separate procedures (measurement of strains in situ, and determination of E) which are time consuming and expensive.
Accordingly, one object of the present invention is to provide a new and improved method and apparatus for measuring the natural state of stress of rock in situ.
Another object of the invention is to provide a new and improved method and apparatus for measuring rock stress as well as other characteristics of the rock, such as Young's Modulus in situ with a single procedure.
A further object of the invention is to provide a new and improved method and apparatus for measuring rock stress at both shallow and deep subterranean regions, wherein orientation of the borehole is not critical.
A further object of the invention is to provide a new and improved method and apparatus for measuring rock stress in a borehole wherein the wall of the borehole is fractured for strain relief.
An additional object of the invention is to provide a new and improved method and apparatus for measuring rock strain wherein a self-aligning apparatus is positioned in the borehole to fracture the wall of the borehole for stress relief, and to obtain initial and relieved strain readings of the rock.
An additional object of the invention is to provide a new and improved, self-contained and reusable instrument for measuring the natural stress of rock in a pre-selected subterranean region.
Yet another object of the present invention is to provide a new and improved instrument, comprising an hydraulic jack for applying self-equilibrating forces to opposite quadrants of the borehole, and measuring values of strain of the rock on the remaining quadrants.